East Asia and Western Pacific METEOROLOGY AND CLIMATE

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the development of an equatorial ITCZ within this region. However, the equatorial ITCZ
subsequently gives way to a monsoon ITCZ that develops near 15 -20°N by the interaction of
the 30-60 day oscillation with the monsoon circulation. Thus there will be an inverse
relationship between the monsoon ITCZ and the equatorial ITCZ. The abrupt northward
movement of the monsoon trough is manifested as a meridional see-saw between the
equator and the monsoon region. Fig. 7 shows the result of a model simulation of the
interaction between the equatorial and the monsoon ITCZ based on the interaction of the
30-60 day oscillations and the three-dimensional monsoon circulation (Lau and Peng, 1989).
The approach of the 30-60 day oscillation to the monsoon region along the equatorial is
signalled by the subsidence motion ahead of the concentrated rising branch of the
oscillation. At this time the whole monsoon region remains quiescent. The second panel at 6
days later, captures the double ITCZ that forms as the rising branch of the the equatorial
30-60 day oscillations is well within the monsoon domain and a monsoon ITCS is generated
at around 15-20°N. Another six days later, the monsoon ITCZ has completely dominated
the convection within the monsoon domain. The equatorial ITCS is suppressed and weak
subsidence appears over the equator.
6. Summary
Based on the above analyses and the examination of a video of GCM 3-hourly IR
images from 1978-1987 (courtesy of the Meteorological Research Institute of Japan), key
features of the East Asian summer monsoon are summarized in Fig. 8.
DRY 2O
0.
\
Q. 0-6
n v
-30 -IS
LRTITUOE
...ft^T/..,
, ! i; i ,
% . „ ... / / • / ^' / '
Figure 7. Vertical streamline pattern showing the vertical structure of the local Hadley
circulation before (Day 6), during (Day 6) and after (Day 12) the passage of the equatorial
intraseasonal disturbances.